flat: add a way to check for existence without reading out the value

[nagisa]

No description provided.

[nagisa]

The coverage here is likely to decrease before #10494 lands with tests.

[codecov]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (20714d5) 72.01% compared to head (3642da6) 71.99%.

Files	Patch %	Lines
core/store/src/flat/storage.rs	5.26%	18 Missing ⚠️
core/store/src/flat/chunk_view.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10510      +/-   ##
==========================================
- Coverage   72.01%   71.99%   -0.03%     
==========================================
  Files         720      720              
  Lines      146459   146480      +21     
  Branches   146459   146480      +21     
==========================================
- Hits       105477   105455      -22     
- Misses      36133    36169      +36     
- Partials     4849     4856       +7     

Flag	Coverage Δ
backward-compatibility	0.08% <0.00%> (-0.01%)	⬇️
db-migration	0.08% <0.00%> (-0.01%)	⬇️
genesis-check	1.25% <0.00%> (-0.01%)	⬇️
integration-tests	36.87% <12.50%> (-0.08%)	⬇️
linux	71.19% <12.50%> (-0.03%)	⬇️
linux-nightly	71.33% <12.50%> (-0.03%)	⬇️
macos	55.11% <12.50%> (-0.01%)	⬇️
pytests	1.47% <0.00%> (-0.01%)	⬇️
sanity-checks	1.27% <0.00%> (-0.01%)	⬇️
unittests	67.96% <12.50%> (-0.01%)	⬇️
upgradability	0.13% <0.00%> (-0.01%)	⬇️

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[Longarithm]

nit: contains_key because it looks classic for rust map structures?

[nagisa]

nit: contains_key because it looks classic for rust map structures?

FWIW as i was working on this change i was perpetually unsure if it is possible for key to be present in the store and hold a None for a value. Hence I went with contains_value just to rule out this distinction. But either way I don't really mind either shade here…

[Longarithm]

if it is possible for key to be present in the store and hold a None for a value.

Fair. It's not possible, e.g. FlatStateValue is the value can be stored in flat state and it doesn't include None. Then, FlatStorage::get_value returns Option<FlatStateValue> which is None if value is not there. I wonder how/where it can be documented better.